Why was Isaac a son of the miracle-working power of God?
Isaac was a son of God's miracle-working power because Abraham was nearing 100 and Sarah nearing 90, both far beyond normal childbearing. God's promise could not fail, so He gave them the son He had named Isaac and established His covenant with him.
Les explains that Isaac was a son of promise, and on top of that he was a son of the miracle-working power of God. Abraham was nearing 100 and Sarah nearing 90, so both were way beyond normal childbearing. Yet God had said He would give them a son, and they were to call his name Isaac. God does not let what He promises fail. Though God also dealt with Ishmael and made him fruitful, the covenant was established with Isaac, whom Sarah would bear at the set time in the next year.